1. Field of the Invention
This invention relates generally to the selective hydrogenation of unsaturated compounds to more-saturated analogs. In particular, alkynes are converted to alkenes by hydrogenation of the alkynes over a fixed bed of supported catalyst, while in an absorptive non-hydrocarbon absorbent.
2. Description of the Related Art
Hydrogenation of alkynes and/or multifunctional alkenes to compounds containing only one alkene group is an important industrial process and is discussed widely in the patent literature. Acetylene, the simplest alkyne, occurs in many processes as a main product or by-product which is thereafter converted to ethylene or ethane by hydrogenation. Thermal cracking of ethane can be caused to produce mostly ethylene, but a minor undesired product is acetylene. Pyrolysis of simple alkanes or mixtures containing primarily alkanes and partial oxidation of simple alkanes or mixtures containing primarily alkanes can be made to produce various blends that contain as principal products both alkenes and alkynes. Products in lower abundance will often include diolefins, compounds containing both yne and ene functionalities, polyenes, and other unsaturated moieties. Most commonly the desired products are the singly dehydrated compounds containing a single ene functionality. Thus, it is desirable to convert the alkynes to alkenes, but not convert the desired alkenes further to alkanes. Reactions of alkenes are generally more controllable than those of alkynes and diolefins, which tend to create oligomers and undesirable polyfunctional compounds.
The hydrogenation step is normally carried out on the primary gas produced in the cracking or pyrolysis reaction of natural gas and low molecular weight hydrocarbons, which includes all the initial gas products, also known as “front-end” hydrogenation, or subsequent to fractionation of the gas components, wherein the only stream subjected to hydrogenation is enriched in the highly unsaturated compounds, also known as “tail-end” hydrogenation. The advantage of primary gas hydrogenation is generally an abundance of the hydrogen required for hydrogenation. However, the excess available hydrogen in front-end hydrogenation can result in “run-away” reactivity wherein conversion of alkenes to alkanes reduces the value of the product. Fractionation reduces the available hydrogen but polymer formation is common, the effect of which is to shorten the useful life of the catalyst.
There are numerous examples of gas-phase hydrogenation of alkynes. For example, U.S. Pat. No. 6,127,310 by Brown, et al. teaches that the selective hydrogenation of alkynes, which frequently are present in small amounts in alkene-containing streams (e.g., acetylene contained in ethylene streams from thermal alkane crackers), is commercially carried out in the presence of supported palladium catalysts in the gas-phase.
In the case of the selective hydrogenation of acetylene to ethylene, preferably an alumina-supported palladium/silver catalyst in accordance with the disclosure in U.S. Pat. No. 4,404,124 and its division U.S. Pat. No. 4,484,015 is used. The operating temperature for this hydrogenation process is selected such that essentially all acetylene is hydrogenated to ethylene (and thus removed from the feed stream) while only an insignificant amount of ethylene is hydrogenated to ethane. Proper temperature selection and control results in minimization of ethylene losses and allows one to avoid a runaway reaction, which is difficult to control.
U.S. Pat. No. 5,856,262 describes use of a palladium catalyst supported on potassium doped silica wherein acetylene ranging in concentration from 0.01% to 5% in blends of ethylene and ethane is converted to ethylene in the gas-phase. U.S. Pat. No. 6,350,717 describes use of a palladium-silver supported catalyst to hydrogenate acetylene to ethylene in the gas-phase. The acetylene is present at levels of 1% in a stream of ethylene. U.S. Pat. No. 6,509,292 describes use of a palladium-gold catalyst wherein acetylene contained in a stream of principally ethylene, hydrogen, methane, ethane and minor amounts of carbon monoxide converts acetylene to ethylene in the gas-phase.
U.S. Pat. No. 6,395,952 describes recovery of olefins from a cracked gas stream using metallic salts and ligands. The cracked gas stream is hydrogenated prior to scrubbing to remove acetylene from the stream.
U.S. Pat. No. 5,587,348 describes hydrogenation of C2 to C10 alkynes contained in comparable streams of like alkenes over a supported palladium catalyst containing fluoride and at least one alkali metal. Examples show hydrogenation of low concentrations of acetylene, below 1%, being converted to ethylene in a gas principally comprised of methane and ethylene at 200 psig and 130° F. and 180° F. Care was taken to avoid heating the gas to a runaway temperature, wherein at least 4.5% of the ethylene would be converted to ethane and the temperature would become uncontrollable, which varied from about 70° F. to 100° F. above the minimum temperature that would reduce the acetylene concentration to acceptable levels.
U.S. Pat. No. 6,578,378 describes a complex process for purification of ethylene produced from pyrolysis of hydrocarbons wherein the hydrogenation follows the tail-end hydrogenation technique. At the top of the de-ethanizer the vapor of the column distillate is treated directly in an acetylene hydrogenation reactor, the effluent containing virtually no acetylene being separated by a distillation column called a de-methanizer, into ethylene- and ethane-enriched tail product. The vapor containing acetylene is exposed to selective hydrogenation to reduce acetylene content of the principally ethylene gas or treated with solvent to remove it and preserve it as a separate product. In all cases the acetylene content of the pyrolysis gas contained less than 1.5 mol % acetylene.
Hydrogenation is also known to occur in the liquid phase where the fluids are easily conveyed or transported as liquids under reasonable temperature and pressure. Naphtha cracking produces significant quantities of C4 and C5 unsaturated compounds, with 1,3 butadiene and 1-butene generally having the greatest commercial value.
U.S. Pat. No. 6,015,933 describes a process in which polymer by-products from the steam cracking of naphtha to butadiene are removed. Acetylenes in the liquid hydrocarbon stream are selectively hydrogenated in a reactor to produce a reactor product containing at least hydrogen, butadiene, and polymer by-products having from about 8 to about 36 carbon atoms, and typically containing butenes and butanes. The acetylenic compounds are primarily vinyl acetylene, ethylacetylene, and methylacetylene. These acetylene group-containing molecules are converted to 1,3 butadiene, 1-butene, and propylene, but can react further with butadiene to form polymeric by-products. The reaction is carried out in the liquid phase with butadiene as the carrier. The undesirable feature of this process is that the carrier reacts with the products of the hydrogenation reaction, necessitating the removal of the polymeric by-products described.
U.S. Pat. No. 5,227,553 describes a dual bed process for hydrogenating butadiene to butenes. This improvement is said to increase selectivity in streams containing high concentrations of butadiene while reducing the isomerization of butene-1 to butene-2, and nearly eliminating the hydrogenation of isobutene to isobutane as well as oligomerization.
U.S. Pat. No. 4,547,600 discloses the need for more silver than previously thought necessary in the hydrogenation of acetylenic compounds that are found in butadiene as a result of steam cracking. The reaction is performed in the liquid phase where the product is the carrier.
U.S. Pat. No. 3,541,178 reports a reduction in the loss of butadiene along with nearly complete reduction of acetylenic compounds by restricting the flow of hydrogen to no more than 80% to 90% of saturation in the hydrocarbon stream. This reduces the potential for polymerization of the vinylacetylenes, as there is no hydrogen remaining in the reaction stream at the end of the reaction. The undesirable aspect of this reduced hydrogen content however, is that the concentration of the hydrogen in the reactor is reduced, which decreases the reaction rate.
U.S. Pat. No. 3,842,137 also teaches a reduction in the loss of butadiene to butene along with nearly complete conversion of vinylacetylene to butadiene, through the use of an inert diluent gas for the hydrogen. The hydrogen-containing gas includes no more than 25% hydrogen. The reaction takes place in the liquid phase, between a temperature of 40° F. and 175° F., and at a pressure of 80 to 200 psig. Again however, an undesirable aspect of using a diluent is that concentration of the hydrogen in the reactor is reduced, which decreases the reaction rate.
U.S. Pat. No. 4,469,907 teaches high conversions of multiply unsaturated hydrocarbons to singly unsaturated hydrocarbons without subsequent isomerization, by staging the insertion of hydrogen into one or more reactors in series. An undesirable aspect of using several reactors however, is the increased complexity of the process, resulting in increased cost and more complicated process control.
There are several examples where non-linear and/or non-hydrocarbon compounds are hydrogenated in the liquid phase. For example, U.S. Pat. No. 5,696,293 describes liquid phase hydrogenation and amination of polyols, carried out at pressures below 20 MPa using a supported ruthenium catalyst and containing another metal from Groups VIA, VIIA, and VIII. A ruthenium-palladium or singly palladium catalyst is listed in the examples. An undesirable feature of this process is the need to filter the fine and expensive catalyst out of the product. Catalyst losses are potentially very costly.
U.S. Pat. No. 5,589,600 discloses hydrogenation of benzene to cyclohexene using ruthenium-nickel catalysts in the presence of water, which is purported to improve selectivity. U.S. Pat. No. 5,504,268 discloses hydrogenation of aromatic acetylenic compounds that are impurities in vinyl aromatic compounds, over a supported palladium catalyst. The purported improvement is obtained via reduction of the hydrogen concentration by using a gas phase diluent such as nitrogen or methane. As previously noted, an undesirable aspect of using a diluent however, is the reduction in the concentration of hydrogen in the reactor and corresponding decrease in the reaction rate.
Carbon monoxide is known to enhance hydrogenation selectivity. It is added to a stream that has been thermally cracked or pyrolized to reduce the hydrogenation of the ene functional groups. U.S. Pat. No. 6,365,790 describes an approach to selective hydrogenation of C10 to C30 alkynes to their respective alkenes in the liquid phase, by careful addition of a compound that decomposes to form CO. An undesirable aspect of using an additive is that the additive must later be removed from the product in diminished form.
U.S. Pat. No. 4,517,395 indicates that CO and H2 added to a liquid phase of C3+ multi-ene or mono-yne hydrocarbons, dispersed in the single-ene containing hydrocarbons, results in improved conversion due to better selectivity. The emphasis is on maintaining sufficient pressure to hold the CO and H2 in the liquid phase rather than dispersed as a heterogeneous phase. Notably, water is added to reduce the amount of CO required as well as to reduce the temperature required.
U.S. Pat. No. 4,705,906 presents a catalyst formulation wherein acetylene is converted by hydrogenation to ethylene, in the presence of CO in concentrations greater than 1 vol % in a temperature range between 100° C. and 500° C. The catalyst is a zinc oxide or sulphide, which may incorporate chromium, thorium, or gallium oxide. Zinc oxide and zinc sulphide were reportedly chosen for the reason that, although palladium catalysts are reasonably tolerant of the usual organic impurities which act solely as activity moderators, palladium catalysts are poisoned at low temperatures by high concentrations of carbon monoxide, such as those associated with unsaturated hydrocarbon-containing products obtained by the partial combustion of gaseous paraffinic hydrocarbons. This is to be contrasted with their behavior at low carbon monoxide concentrations, typically at concentrations less than 1 vol %, at which moderation of catalytic activity is reported to enhance the selectivity of acetylene hydrogenation to ethylene. At high temperature, palladium catalysts are active even in the presence of carbon monoxide, but selectivity of acetylene hydrogenation to ethylene is drastically reduced by simultaneous hydrogenation of ethylene to ethane.
In U.S. Pat. No. 4,906,800, a Lindlar catalyst was used with a feed that contained no CO. The catalyst contained 5% palladium on a CaCO3 support with about 3% lead as a promoter. After special treatment involving oxidation, reduction in CO, and finally a heat treatment step of the readily oxidized and reduced Lindlar catalyst, the treated catalyst showed improved selectivity at high conversion, but again at higher temperatures above 200° C. selectivity decreased significantly.
U.S. Pat. No. 5,847,250 describes a supported palladium catalyst employing a “promoter” from Groups 1 or 2 (in the New classification system; CAS Groups IA and IIA) and the palladium being supported on silica that has been pretreated to contain the promoter. The purported advantage is that no carbon monoxide is needed to provide increased selectivity because the selectivity-increasing effect of the carbon monoxide is strongly temperature dependent. Large temperature gradients in the catalyst bed therefore have an adverse effect on the selectivity when carbon monoxide is present. The reaction is performed in the gas phase in one or more beds with or without intermediate cooling or hydrogen gas addition. Acetylene content ranges from 0.01% to 5%. The reported selectivity ranges from 19 to 56%.
The use of liquid carriers has also been described in several patents for various reasons. For example, U.S. Pat. No. 4,137,267 describes the hydrogenation of alkyl aminoproprionitrile in the liquid phase, using hydrogen and ammonia as reactants over a supported catalyst and using an organic solvent. The solvent was selected to absorb excess heat by vaporizing at the process conditions, which is said to provide some temperature control. An undesirable aspect of employing a volatilizing solvent is the concomitant difficulty of employing this technique in a fixed catalyst bed.
U.S. Pat. No. 5,414,170 teaches selective hydrogenation of a stream from an olefin plant after operation of a depropanizer but prior to operation of a de-ethanizer or de-methanizer. The hydrogenation is performed on the mixed-phase propane rich ethylene stream, as well as subsequently on the vapor product. A method is described by which the acetylenes in the front end of an olefin plant process stream are hydrogenated in the presence of a liquid hydrocarbon. The propane liquids, initially separated out of the inlet process stream, are used later to cool and wash the product of the acetylene hydrogenation reactor by adding them to the acetylene-containing stream during hydrogenation. An undesirable aspect of this process is the need to fractionate the propane from the small amount of ethylene produced.
U.S. Pat. No. 5,059,732 discloses a process to hydrogenate effluent from a steam cracker containing ethylene, acetylene, propylene, propyne, propadiene, and butadiene, with hydrogen in the presence of a palladium or other noble metal catalyst by use of a gasoline cut as an inert carrier. The rationale for improved catalyst life is that the aromatic content of the gasoline carrier prevents plating out of the diolefins on the catalyst, which can otherwise polymerize and form gums that obstruct the other reactive components from getting to the catalyst surface. An undesirable aspect of this process however, is the need to fractionate the heavier hydrocarbon fraction from the small amount of ethylene produced, although this is not as serious a problem as when propane is used as the carrier.
Several patents disclose the use of solvents to separate the acetylenic fraction of a fluid stream from the other components. It is well known that dimethylformamide (DMF) and n-methyl-2-pyrrolidone (NMP) are good liquid absorbents for acetylene. Likewise, it is well known that DMF, furfural, ethylacetate, tetrahydrofuran (THF), ethanol, butanol, cyclohexanol, and acetonitrile are useful absorbents for 1,3-butadiene.
French Patent No. 2,525,210 describes a method for the purification of a stream containing mostly ethylene with a smaller amount of acetylene contaminant, wherein the acetylene is not converted to ethane. The basic concept is to hydrogenate a gas stream short of complete conversion, leaving some acetylene in the gas stream, then to absorb the acetylene in a solvent that extracts the acetylene from the ethylene stream. This extracted acetylene is separated from the solvent and recycled to the ethylene stream for hydrogenation. This is said to increase conversion to ethylene. An undesirable aspect of this process is the need to control the hydrogenation significantly below complete conversion.
U.S. Pat. No. 4,277,313 focuses on the purification of a C4 stream containing acetylenic compounds by hydrogenation of the acetylenic compounds followed by downstream separation. The hydrogenation step is carried out in the liquid phase after the hydrocarbon has been separated from the absorbing solvent. It is said to be important to remove the acetylenic compounds prior to polymerization since they can form explosive metal acetylides and will cause the polymer to be off-spec. Suitable inert solvents for this process purportedly include: dimethylformamide (DMF), furfural, ethylacetate, tetrahydrofuran (THF), ethanol, butanol, cyclohexanol, and particularly acetonitrile.
U.S. Pat. No. 3,342,891 describes fractionating a stream of C4 and C5 alkadienes into two streams, where one stream is reduced in vinyl acetylenes and the second stream is enriched in vinyl acetylenes. DMSO was used to separate the vinylacetylene from the enriched stream. The DMSO that contains the vinylacetylene was stripped with nitrogen to conentrate the vinylacetylene, which was subsequently hydrogenated in the gas phase. Unconverted vinyl acetylene in the effluent is recycled back to the feed of the fractionation column.
In some examples, the use of a liquid carrier or solvent is disclosed in which the liquid carrier or solvent is present during the hydrogenation step. U.S. Pat. No. 4,128,595 for example, teaches a process wherein gaseous acetylene or acetylene group containing compounds are contacted with hydrogen via an inert saturated liquid hydrocarbon stream with hydrogenation occurring over a typical Group VIII metal supported on a catalyst medium. Examples of inert saturated hydrocarbons include various hexanes, decanes and decalin. The process requires the acetylene containing compound and saturated hydrocarbon solvent be fed co-currently into the top of a trickle bed reactor because the solubility of the acetylene containing compound in the saturated hydrocarbon solvent is poor at reaction conditions. An undesirable aspect of this process is the poor solubility of the hydrocarbon solvent toward acetylene. This patent teaches that rapid catalyst deactivation can occur with polar solvents. Dimethylformamide (DMF) was used as an absorbent for ethylene and the polar solvent during hydrogenation. The result indicated rapid catalyst deactivation with conversion dropping from 100% to 50% over a period of 17 hours. Accordingly, there is substantial need for a practicable liquid phase hydrogenation process, using satisfactory non-hydrocarbon solvents, if sufficient selectivity and conversion can be provided.
As is apparent, an efficient, practicable process for liquid-phase selective hydrogenation, with sufficient conversion and selectivity, would be a substantial contribution to the art. It has now been found that high alkyne conversion can be obtained with significant improvement in the selectivity to the corresponding alkene relative to the alkane in accordance with the present invention. Surprisingly, and contrary to the teachings of the conventional art relating to use of a polar solvent, such as dimethylformamide, a progressive decline in catalyst activity with time on stream is not observed with the inventive process, and excellent selectivity is obtained. In particular, the present invention exhibits increasing acetylene conversion to high steady state values, while exhibiting excellent selectivity to ethylene, with sustained performance in operation for extended periods of time.